1. Field of the Invention
The apparatus of the present invention relates to catheter fluid-velocity flow probes of the electromagnetic type, all of which may be referred to as flow probes.
2. Description of the Prior Art
In recent years there has been much research and development in the art of catheter devices suitable for insertion into the body of a human or animal subject to measure various functions of body fluids. For example, in U.S. Pat. Nos. 3,724,274 and 3,748,623 to Millar there are disclosed pressure transducers suitable for attachment to a catheter for implantation into an artery or vein to measure the blood pressure of a person at a given point.
In U.S. Pat. No. 3,516,399 to Barefoot there is disclosed an electromagnetic catheter blood flow probe suitable for attachment to the distal end of a catheter. When the catheter containing the blood flow probe is inserted into an artery or vein, velocity of blood in that artery or vein may be measured.
The blood flow probe disclosed by Barefoot comprises an electromagnet consisting of a U-shaped iron core onto which is wound a suitable number of turns of wire. When a voltage is applied between the ends of the wire, a magnetic field is created between the open ends of the U-shaped iron core. Electrodes are disposed within the magnetic field that is created in the probe tip, and, when blood flow past these electrodes, a voltage is induced across them. Conductors connect the electrodes to a connector attached to the proximal end of the catheter.
The voltage induced across the electrodes is directly proportional to the number of magnetic flux lines that the blood is able to intersect as it flows by the probe. It is known that the number of magnetic flux lines emerging from the electromagnet may be increased by increasing the number of turns of wire on the iron core. As the number of turns of wire on the iron core is increased, however, the electrical resistance of the electromagnet increases. Consequently, the amount of heat generated in the probe increases. It should be apparent that an excessive amount of heat dissipated at the probe tip can cause damage to a blood vessel or to the blood.
It would be desirable to construct a multi-function catheter probe so that multiple measurements and/or samples could be taken with a single catheterization. For example, it might be desirable to have a catheter having both a flow probe and a pressure transducer attached to the distal end of the catheter. One such flow probe is the one shown in an article entitled "Sensitivity of Electromagnetic Velocity Probes", Phys. Med. Biol., Vol. 16, No. 2, pp. 229-232 (1971). This flow probe employs a coil without a ferromagnetic core. Since there is no ferromagnetic core, sufficient cross-sectional area is available in the probe to provide a central lumen. The proximal end of the lumen in the flow probe may be attached to the distal end of the catheter, and a suitable pressure transducer may be attached to the lumen at the distal end of the probe.
It has been found that the flow probe described in the preceeding paragraph: (1) becomes very hot in operation; and (2) lacks sufficient sensitivity for reliable measurements. Both problems are believed to be directly attributable to the coil requirements in the absence of a ferromagnetic core.
The Carolina-Millar catheter, which employed multiple measurement devices attached to its distal end, is shown and discussed in Review of Surgery, Vol. 29, No. 2, p. 149, Mar.-Apr. 1972. The measurement devices employed were the blood flow probe with a ferromagnetic core disclosed by Barefoot in U.S. Pat. No. 3,516,399 and the pressure transducer disclosed in U.S. Pat. No. 3,724,274 to Millar. In this device a passage for wires was provided between the proximal end of the catheter and the pressure transducer at the distal tip.
The size of the core of the electromagnet in the Carolina-Millar catheter was substantially reduced to provide sufficient cross-sectional area for the passage of wires to the pressure transducer, and a reduction of the sensitivity of the blood flow probe was observed. In other words, the performance of prior art flow probes with ferromagnetic cores deteriorated when a passage was provided for the attachment of multiple measurement devices to the distal end of the probe.
Accordingly, it is an objective of this invention to provide a catheter fluid-velocity flow probe which is adaptable to have other attachments added thereto without reducing probe sensitivity. Such attachments may include, for example, a pressure tranducer attachment, a fiber optic attachment, or a blood sample gathering attachment.
Furthermore, it is another objective of the present invention to provide a catheter fluid-velocity flow probe which has greater sensitivity and dissipates less heat in operation than do blood flow probes of the prior art.